exist-open

Is this a valid range index configuration?
<create qname="@lastUpdateDt" type="xs:dateTime"/>
<create qname="@lastUpdateDt" type="xs:string"/>
I have some queries that treat this value as a string, and others that treat it as a dateTime. If I define the indexing this way, will the XQuery pick whichever one is most appropriate?
We're experimenting with it, but I was hoping someone could tell me whether or not this is actually supported.
Thanks!
Jason Smith